To save water and improve sewage drainage effects of the water closet, a patent with an application number of 201010126268.1 disclosed a “buoyancy control mechanism of a suction drainage device of a water closet”; the mechanism is connected to the suction drainage device, and drives suction and sewage drainage by the buoyancy. Existing problems are that a buoyancy tank has a large volume and a small acting force; because of the size of the water closet, the installation position and operable space are limited, and the volume of the buoyancy tank needs to be increased for improving the suction force, thereby limiting technical promotion and application of the “buoyancy control mechanism of a suction drainage device of a water closet”.